An optical mode multiplexer was newly designed and fabricated using LiNbO3 waveguides. The multiplexer consists of an asymmetric directional coupler capable of achieving the phase-matching condition by the voltage adjustment. The mode conversion efficiency between TM0 and TM1 modes was quantitatively measured to be 0.86 at maximum.

A Mach-Zehnder optical modulator with the tunable multimode interference coupler was fabricated using Ti-diffused LiNbO3. The modulation extinction ratio could be voltage controlled to maximize up to 50 dB by tuning the coupler. Optical single-sideband modulation was also achieved with a sideband suppression ratio of more than 30 dB.

Optical phase conjugation (OPC) is an all-optical signal processing technique for mitigating fiber nonlinearity and is promising for building cost-efficient fiber networks with few optic-electric-optic conversions and long amplification spacing. In lumped amplified systems, OPC has a little nonlinearity mitigation efficiency for nonlinear distortion induced by cross-phase modulation (XPM) due to the asymmetry of power and chromatic dispersion (CD) maps during propagation in transmission fiber. In addition, the walk-off of XPM-induced noise becomes small due to the CD compensation effect of OPC, so the deterministic nonlinear distortion increases. Therefore, lumped amplified transmission systems with OPC are more sensitive to channel spacing than conventional systems. In this paper, we show the channel spacing dependence of NZ-DSF transmission using amplification repeater with OPC. Numerical simulations show comprehensive characteristics between channel spacing and CD in a 100-Gbps/λ WDM signal. An experimental verification using periodically poled LiNbO3-based OPC is also performed. These results suggest that channel spacing design is more important in OPC-assisted systems than in conventional dispersion-unmanaged systems.

We evaluated the noise properties of a periodically poled lithium niobite phase-sensitive amplifier (PSA) using a phase-locked local oscillator as a pump generated by an optical phase-locked loop (OPLL-LO). To examine whether or not the LO pump generated by an OPLL degrades the noise figure (NF) of the PSA, we compared the noise levels of a PSA using an OPLL-LO with that of one using a master local oscillator (M-LO) that utilizes the master light itself as a pump in the electrical domain. With the OPLL, the phase-locked local light had almost the same frequency noise components as the master light. We observed almost the same output noise spectra for the OPLL-LO PSA and M-LO PSA and confirmed the absence of excess noise components in the OPLL-LO PSA in the 0.1 to 20-GHz range. The OPLL-LO PSA exhibited low-noise amplification with an average NF of 1.7-dB at a 23.2-dB gain within an input power range of -31 to -21dBm, which is a feasible input power for repeater amplifiers used in the optical signal transmission systems. We also investigated the influence of the noisy master light, which emulates the accumulation of optical noise from the amplifiers in the transmission system. The OPLL-LO PSA was highly tolerant to the optical noise because the difference in the NF was negligibly small within a master light OSNR range of 5 to 55dB. These results indicate that the OPLL-LO PSA will be useful as a low-noise repeater amplifier for the spectrally efficient large-capacity photonic networks of the future.

We developed a polarization-independent and reserved-band-less complementary spectral inverted optical phase conjugation (CSI-OPC) device using dual-band difference frequency generation based on highly efficient periodically poled LiNbO3 waveguide technologies. To examine the nonlinearity mitigation in a long-haul transmission using a large number of OPCs, we installed a CSI-OPC device in the middle of a pure silica core fiber-based recirculating loop transmission line with a length of 320km. First, we examined the fiber-input power tolerance after 5,120-km and 6,400-km transmission using 22.5-Gbaud PDM-16QAM 10-channel DWDM signals and found a Q-factor improvement of over 1.3dB along with enhanced power tolerance thanks to mitigating the fiber nonlinearity. We then demonstrated transmission distance extension using the CSI-OPC device. The use of multiple CSI-OPCs enables an obvious performance improvements attained by extending the transmission distance from 6,400km to 8,960km, which corresponds to applying the CSI-OPC device 28 times. Moreover, there was no Q-factor degradation for the link in a linear regime after applying the CSI-OPC device more than 16 times. These results demonstrate that the CSI-OPC device can improve the nonlinear tolerance of PDM-16QAM signals without an excess penalty.

We propose a highly sensitive carrier-recovery system for in-line amplification for binary phase shift keying (BPSK) signals in a periodically poled LiNbO3 based phase sensitive amplifier (PSA). We applied a discrete two-stage second harmonic generation/difference frequency generation (SHG/DFG) parametric conversion scheme to enhance the sensitivity of the carrier recovery. Owing to this two-stage SHG/DFG scheme, the conversion efficiency of the seed light for the injection locking needed for the pump generation can be improved compared to that of the cascaded SHG/DFG scheme. The new discrete scheme might also prevent the SNR degradation of the seed light caused by the ASE from the booster EDFA compared with the previous system that used the cascaded scheme. This novel carrier-recovery system exhibits high sensitivity with the SNR of over 7.8dB of the seed light, while tapped signal power is as low as -50dBm, which is low enough for injection locking. The new in-line PSA with this carrier-recovery system exhibits high gain of over 11dB. Since we successfully obtained the high gain property, we tried multistage amplification taking into account practical use and achieved it with both a high gain of 20dB and a noise figure that is almost as low as the standard quantum limit of a PSA.

A millimeter-wave radar receiver using a z-cut LiNbO3 optical modulator with orthogonal-gap-embedded patch-antennas on a low-k dielectric material is proposed. A millimeter-wave from a reflected radar signal can be received by the patch-antennas and converted directly to a lightwave through electro-optic modulation. A low-k dielectric material is used as a substrate for improving antenna gain. Additionally, an interaction length between millimeter-wave and lightwave electric fields becomes long. As a result, large modulation efficiency can be obtained, which is proportional to sensitivity of the millimeter-wave radar receiver. Optical millimeter-wave radar beam-forming can be obtained using the proposed device with meandering-gaps for controlling interaction between millimeter-wave and lightwave electric fields in electro-optic modulation. Analysis and experimentally demonstration of the proposed device are discussed and reported for 40GHz millimeter-wave bands. Optical millimeter-wave radar beam-forming in 2-D is also discussed.

We demonstrate high baud-rate DQPSK modulation with full-ETDM technique using a novel high-speed optical IQ modulator consisting of a ridge-type optical waveguide structure on a thin LiNbO3 substrate. Our fabrication technique achieves a drastic extension of the modulator's bandwidth and a reduction of half-wave voltage. Demonstration of 90-Gbaud NRZ-DP-DQPSK signal generation with the modulator successfully achieved a bit rate of 360-Gb/s under full-ETDM configuration.

For the uplink of a radio-on-fiber system or an electromagnetic field sensor, a resonant type optical modulator array connected with antennas can effectively convert a micro/millimeter-wave to a light wave. We designed and fabricated 10 GHz band resonant modulators and micro-strip antennas. And we demonstrated the simultaneous operation of four modulators using power received by micro strip antennas connected to each modulator. We confirmed that the optical phase change induced by the received power could be proportionally increased with the number of arrays.

This paper describes improvement of the gain in the Er-doped lithium niobate waveguide optical amplifiers. A new configuration is proposed, which is loaded with a high-index cladding for the purpose of realizing a larger overlap between the guided light fields and doped Er ion-concentration. It is shown by theoretical simulations and also by experiments that the clad is advantageous to shifts of the light fields toward the substrate surface, and that the overlap between the light field and Er ion-concentration becomes large. Improved optical gains are measured for a fabricated device with a thin-TiO2 clad in the LiNbO3 substrate.

12 on/off power splitters at λ=0.63µm have been produced in LiNbO3 substrates using strain-induced channel waveguides formed by magnetron deposition of surface metal films and lift-off technology. The static strain resulting from thermal expansion mismatch between the substrate and the metal films induces a localized increase in the refractive index via the strain-optic effect. On/off voltage of about 25V has been demonstrated.

In this paper, we report on our recent work in designing and developing an optical waveguide and optical integrated circuit for optical BFN in adaptive multibeam array antenna. We introduce a new integrated Ti:LiNbO3 waveguide and prove that it is able to yield large birefringence and birefringence dispersion. We present a new technique using a microwave-modulated optical wave to measure the birefringence in integrated Ti:LiNbO3 optical waveguides. The measuring results show that the new waveguide has a birefringence of 0.08 and birefringence dispersion of 0.05 µm-1 at optical wavelength of 1.55 µm. When the new Ti:LiNbO3 is applied to form a integrated optical waveguide array in optical beamforming network, it is shown that microwave phase shifts within the range of [-180, +180] is achieved by tuning the optical wavelength 10 nm around 1.55 µm.

The 1 4 optical switch array for wavelength division multiplexing system has been demonstrated where four 14 waveguide digital optical switches have been integrated into one chip. Tape fiber is used as the connection between switch modules to avoid an interconnection fiber jungle. The architecture can be adapted to any number of wavelength channels. Redundant optical switch stage for crosstalk rejection has been used to attain a low crosstalk level. Electro-optic switching is used to attain low power consumption indispensable for routing large wavelength channels.

1.5 µm-band LiNbO3 quasiphase matched (QPM) wavelength converters consisting of a periodical domain inverted structure and a proton exchanged waveguide, have been studied in detail both theoretically and experimentally. Optimum device fabrication conditions are investigated with respected to waveguide propagation loss, coupling loss to a single-mode fiber and wavelength conversion efficiency. A normalized conversion efficiency as high as 200 %/W (by a SHG measurement) and a fiber-to-fiber insertion loss of less than 3.5 dB (@1.55 µm) is obtained for a wavelength converter module with a device length of 40 mm. It is shown that a highly uniform periodical domain inverted structure and a uniform proton exchange waveguide are key to obtaining efficient wavelength conversion. The tolerance of the waveguide width fluctuation is found to be very critical and is less than 20 nm for a 40 mm-long device. Future optimization of LiNbO3 QPM wavelength converters and the possible device applications in future optical communication systems are also presented.

State of the arts on guided-wave optical switch arrays are reviewed. In this paper, electro-optic Ti:LiNbO3 devices are mainly described in comparison with crosspoint switch element structures and switch array architectures. Packaging technologies and stability problems are discussed for practical system applications. Recent development on other materials such as semiconductor waveguides, thermo-optic glass/polymer waveguides are also reviewed briefly.

A large scale optical switch array based on guided-wave technology using banyan network architecture is demonstrated. Banyan network architecture is the simplest NN network connecting a input port to all the output ports. A banyan network optical switch array serves as a base for constructing many classes of switch networks, as we propose in this report. We fabricated a 3232 switch and measured its characteristics. Drive voltage was about 12 V and extinction ratio was 18 dB, and the average insertion loss was 18 dB. Preliminary experiments were conducted on a 6464 device. The use of proton exchanged waveguides makes a 10 mm radius of curvature feasible.

State of the arts on guided-wave optical switch arrays are reviewed. In this paper, electro-optic Ti:LiNbO3 devices are mainly described in comparison with crosspoint switch element structures and switch array architectures. Packaging technologies and stability problems are discussed for practical system applications. Recent development on other materials such as semiconductor waveguides, thermo-optic glass/polymer waveguides are also reviewed briefly.

A large scale optical switch array based on guided-wave technology using banyan network architecture is demonstrated. Banyan network architecture is the simplest NN network connecting a input port to all the output ports. A banyan network optical switch array serves as a base for constructing many classes of switch networks, as we propose in this report. We fabricated a 3232 switch and measured its characteristics. Drive voltage was about 12 V and extinction ratio was 18 dB, and the average insertion loss was 18 dB. Preliminary experiments were conducted on a 6464 device. The use of proton exchanged waveguides makes a 10 mm radius of curvature feasible.

This paper describes the design, fabrication, and performance of a novel Ti:LiNbO3 optical modulator with a two-stage coplanar waveguide electrode for 40 Gbit/s optical transmission systems. The structure consists of a thin lower electrode and a thick upper electrode in conjunction with a ridge structure. The lower electrode ensures low voltage and the upper layer provides good microwave characteristics. Based on simulation results, a fully-packaged module was fabricated. The measured 3-dB electrical bandwidth is 30 GHz with a half-wave voltage of 2. 9 V.

The microwave attenuation, which is the key factor for realizing very large bandwidths Ti:LiNbO3 optical modulators is fully studied and the causes and reduction techniques are discussed in detail. Practical realization of wide-band optical modulators with low microwave attenuation and low driving voltage is also discussed.